Sky Twin

An empty celestial sphere, viewed from inside it. Extends Astro Twin; the subject here is the sky itself rather than anything orbiting in it, so the camera lives inside the sphere and cannot orbit out of it — maxDistance stops short of the 400-unit sky radius, because seeing your own sky from the outside gives the whole illusion away. celestial.loadSky fetches everything at once, or load the three pieces separately. The STARFIELD is the real naked-eye sky: about 9000 stars to magnitude 6, each coloured from its catalogued B-V index through a blackbody ramp, so Rigel is blue-white, Arcturus amber and Betelgeuse orange because that is what their colour indices say and not because anyone picked. Size comes from apparent magnitude on a chart-style compressed scale, so Sirius dominates without the faint majority vanishing. CONSTELLATION FIGURES arrive as one object PER constellation, keyed by IAU three-letter code, and that is the point: a legend entry can toggle Orion by itself, and `only: ['Ori','UMa']` loads a handful instead of all 88. DEEP-SKY OBJECTS are the bright catalogue — the Messier galaxies, nebulae and clusters — placed at their real coordinates, sized by real angular extent and coloured by type so a legend can group galaxies apart from nebulae. A graticule is available in either frame: equatorial, or ecliptic if you want to see the plane the planets keep to. The frames matter, and are handled once rather than per object. Catalogue coordinates are equatorial, orbits are ecliptic, and the two differ by the 23.44-degree obliquity — so the whole sky sits in one group carrying a single rotation about the vernal equinox, which both frames share, instead of every star being rotated on its own. That is also why a marker placed by RA/Dec lands on the same tilted sphere the stars do: the celestial:placement service puts it through that group's own matrix, which is the same mechanism by which a marker anchors to a lat/lon on terrain. Hovering reports the RA/Dec under the pointer, so a plate can show a live readout the way Terrain Twin shows elevation. For a twin, this is the observatory plate. A telescope's pointing becomes a marker at an RA/Dec that moves with the feed; seeing, cloud cover, dome and mount status become gauges and LEDs; a target list becomes markers you can toggle as a group from the legend; a transit or variable-star light curve becomes a sparkline with a threshold rule behind it. The star-temperature scalarbar inherited from Astro Twin has a real range to show as soon as a starfield is loaded — switch it on from Studio's Scalar bars section.